RF coil for imaging system

ABSTRACT

An RF coil suitable for use in imaging systems is provided which coil has a dielectric filled cavity formed by a surrounding conducting enclosure, the conducting enclosure preferably being patterned to form continuous electrical paths around the cavity, each of which paths may be tuned to a selected resonant frequency. The patterning breaks up any currents induced in the coil and shortens path lengths to permit higher frequency, and thus higher field strength operation. The invention also includes improved mechanisms for tuning the resonant frequency of the paths, for selectively detuning the paths, for applying signal to the coil, for shortening the length of the coil and for controlling the field profile of the coil and the delivery of field to the object to the image.

RELATED APPLICATIONS

This application claims the benefit of U.S. patent application Ser. No.09/575,384, filed May 22, 2000, entitled “RF Coil for Imaging System,”by J. T. Vaughan, which application claims the benefit of U.S.Provisional Patent Application Serial No. 60/135,269, filed May 21,1999, entitled “RF Coil for Imaging System and Use Therein,” by J. T.Vaughan, each of which is incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to imaging systems employing radio frequency (RF)coils for RF field generation, and more particularly to RF coils for usein such systems which coils facilitate higher frequency, higherefficiency, higher energy operation, permit use of larger coils,facilitate flexibility in coil design to accommodate a variety ofapplications and provide enhanced signal-to-noise performance so as toachieve among other things improved MRI, fMRI and MR spectroscopicimaging, all the above being achieved without significant increase incost. The invention applies similarly to EPR or ESR.

BACKGROUND OF THE INVENTION

Nuclear magnetic resonance (NMR) or magnetic resonance imaging (MRI),functional MRI (fMRI), electron spin resonance (ESR) or electronparamagnetic resonance (EPR) and other imaging techniques using RF fieldgenerating coils are finding increasing utility in applicationsinvolving imaging of various parts of the human body, of otherorganisms, whether living or dead, and of other materials or objectsrequiring imaging or spectroscopy. For purposes of this application, RFshall be considered to include frequencies from approximately 1 MHz to100 GHZ, the upper ranges of which are considered to be microwaves.While existing such systems are adequate for many applications, there isoften a need for higher signal-to-noise and improved spectral resolutionin such imaging so as to permit higher spatial resolution, higher imagecontrast, and faster imaging speed. In fMRI applications for example,where multiple images may be taken over time and a difference imagegenerated to permit visualization of small changes in blood oxygen useover time in the body being imaged, differences between successiveimages may be very small, requiring high signal-to-noise to permitdetection. A major limitation to higher resolution, and/or fasterimaging is an insufficient signal to noise ratio. If the image signalintensity is below the noise level, an image can not be made. It istherefore important in high resolution systems to design an RF coil tomaximize signal and to minimize noise. The RF coil of such a systemshould also be designed to minimize eddy currents propagating thereinwhich are induced by time transient currents in gradient coils or byother causes.

The signal-to-noise ratio (SNR) and spectral resolution are increased byincreasing the magnetic field strength of the system, generallyexpressed in tesla (T). The SNR benefits of image speed, spatialresolution, and contrast are also increased with the magnetic fieldstrength. However, the frequency of which the nuclei of atoms in thebody resonates varies as a function of the applied magnetic field, witheach atomic species having a unique magnetic field dependent resonantfrequency referred to as the Larmor frequency. For the human body whichis composed primarily of hydrogen atoms in water, fat and muscle tissue,these hydrogen nuclear (proton) frequencies are approximately 64 MHZ fora field strength of 1.5 T, 170 MHZ (4 T), 175 MHZ (4.1 T), 300 MHZ (7T), 340 MHZ (8 T) and 400 MHZ (9.4 T). Other species of atomic nuclei ina body would resonate at other frequencies for a given field strength.However, while conventional birdcage coils in existing MRI and relatedsystems might resonate at a frequency of 170 MHZ (4 T) for example, theconventional birdcage coil with lumped elements (reactance) will operatevery inefficiently, radiating much of its energy like an antenna, ratherthan conserving its energy like a “coil”. At higher frequencies still,such lumped element coils of human head or body dimensions will notreach the Larmor resonant frequency required, limiting the magneticfield strength at which such MRI or EPR systems can operate. Further,since frequency is a function of the electrical path lengths (measuredin wavelengths) in the RF coil, higher frequency, and thus higher fieldstrength operation, has been previously achievable only with very smallcoils which are not always useful for imaging a human being or otherlarger objects. A need therefore exists for an RF coil design whichprovides short electrical path lengths and shields against radiativelosses, while still permitting an RF coil to be constructed withphysical dimensions sufficient to image a human body and/or other largerobjects with high frequency RF energy, thus permitting high fieldstrength operation. It is also desirable to be able to tune each path ofan RF coil to a precise resonant frequency, to be able to provide two ormore resonant frequencies for different paths on the coil, and to beable to easily adjust/retune the resonant frequency of a path or paths.

Still another potential problem in operating these imaging systems,especially at high fields, is in driving the RF coil in a manner so asto achieve a homogeneous RF field, even when a body is positioned in thefield, or to achieve some other desired field profile. Many factorsinfluence field profile or contours including the manner in which thecoils are driven, the geometric and frequency dependent electricalproperties of the anatomy or object, and the frequency dependentproperties of the coil circuits. Techniques for controlling these andother factors to achieve a homogenous or other desired field profile aretherefore desirable. Also, while in many systems the same coil is usedfor both the transmitting of RF energy and the receiving thereof, thecoils being switched between transmit and receive circuitry, there aremany applications where the homogeneous excitation of NMR signal isachieved with a large volume coil and a small local receive coil havingvery short path lengths is used for achieving high SNR operation, suchlocal receive coil being placed as close to the region of the body beingimaged as possible. However, having both the large transmit coil and thelocal RF receive coil tuned to the same frequency results in the coilsbeing destructively coupled (by Lenz's Law for example), this defeatingenhanced operation from the local receive coil. It is thereforedesirable to be able to quickly detune the large RF transmit coil duringa receive operation by a local RF receive coil and vice versa. Improvedways of achieving this objective, particularly in an RF coil providingthe characteristics previously indicated, are therefore desirable.

Finally, some of the advantages of having a local receive coil, and inparticular the ability to place the RF coil closely adjacent to a regionwhere imaging is desired, could be achieved if the RF coil were designedso as to localize both the transmission and reception of RF energy.While coils adapted for performing this function in certain specializedsituations have existed in the past, a more general purpose design forRF coils to facilitate their use in producing localized RF fields andthe localized reception of RF (NMR) signal, is desirable in order toachieve the enhanced SNR benefits of higher image signal, resolution,speed and contrast.

While some of the advantages indicated above are achieved by distributedimpedance RF coils disclosed in U.S. Pat. Nos. 5,557,247 and 5,744,957,which patents have the same inventor as this invention, the systemstaught in these patents, and in particular the RF coils thereof, do notprovide optimum performance in all situations, and improvements arepossible on various aspects of these RF coils, including eddy currentsuppression, design of the coil for optimum positioning in a greaternumber of cases, improved control of field profile, improved tuningoptions and improved detuning in situations where the use of two or morecoils is desired.

SUMMARY OF THE INVENTION

In accordance with the above, this invention provides an RF coil for usein an imaging system, which coil has a cavity formed as a conductiveenclosure in which resonant field can be excited, the enclosure beingformed at least in part of an electrical conductor patterned to form RFconductive paths around the cavity. At least one tuning mechanism may beprovided which determines a resonant frequency or frequencies for suchpaths. The tuning mechanism may be fixed, resulting in a preselectedresonant frequency for the path, or variable to provide a tunableresonant frequency or frequencies. The tuning mechanism may reactivelyadjust the electrical length of each path to tune the path. The pathreactance may also be adjusted to achieve a selected field profile forthe coil. The tuning mechanism may tune all the paths to the sameresonance frequency or may selectively tune the paths to resonate at twoor more different frequencies. In particular, alternate ones of thepaths may be tuned to resonate at a first frequency and the remainingones of the paths tuned to resonate at a second frequency.

The coil may also include a dielectric which at least substantiallyfills the cavity. the thickness of the conductor for at least selectedportions of the enclosure may be substantially greater than one skindepth at the resonant frequency, the dielectric filling the cavityhaving a dielectric constant different from that of air. This results insignals of different frequencies propagating on the outer and innersurfaces of the conductor.

Each of the N paths on the coil may have at least one non-conductive gapformed therein, and the tuning mechanism may include a reactance and/oran impedance across at least selected ones of said gaps. Thereactance/impedance for at least selected ones of the gaps may bevariable to control the resonant frequency for the corresponding path.The reactance for at least some embodiments includes a capacitor, thecapacitance of which may be varied and/or an inductor the inductance ofwhich may be varied. The variable impedance and/or reactance may becontrolled to tune, retune and/or detune the path in which it islocated. Where the enclosure is formed of an outer wall, inner wall, andside walls, end conductive lands may be formed on each of the walls,with corresponding lands on each wall being connected to form the pathsand the gaps being formed in the conductor for each of the paths for atleast one of the walls. For some embodiments, the gaps are formed in theouter wall conductor for each path.

The resonant frequency of the paths may be determined by distributedcapacitance and distributed inductance for the path, the distributedcapacitance being determined by the area of the electrical conductor foreach path, a dielectric fill material for the cavity and/or thedimensions of the dielectric fill material. The electrical conductorforming each path may be a thin foil, the distributed inductance for thepath being a function of the path length. At least one reactancecomponent may be provided in at least selected ones of the paths, thereactance component being either distributed or lumped. A distributed ordiscrete reactance may be selected to achieve a desired resonantfrequency for the paths. The paths have a cumulative reactance whichincludes at least in part the distributed capacitance/inductance, thecumulative reactance for the paths being tuned to result in D differentresonant frequencies for the coil, every Dth path symmetrically spacedaround the coil being tuned to the same frequency.

The coil may include a circuit which applies RF signal to and/orreceives RF signal from M selectively spaced ones of said paths, where Mis an integer and 1≦M≦N. The RF signals may be phase shiftedcorresponding to a phase shift for the corresponding paths to providecircular or other polarization for the coil. Each RF signal ispreferably reactively coupled to the corresponding path, the couplingreactance for each path being variable for some embodiments toindependently match/tune the path. In particular, the couplingreactances may be impedance matched to different loading conditions forthe coil. For some embodiments, the RF coil may be used to transmitand/or receive RF signals, but not both simultaneously, and includes adetuning mechanism for the paths, the detuning mechanism being operativewhen the RF coil is not in the one of the transmit/receive modes forwhich it is being used. The detuning mechanism may include a mechanismfor altering the path length and/or impedance for each path to bedetuned, and in particular may include for some embodiments a PIN diodecircuit for each path which facilitates rapid switching to a changedimpedance state sufficient to effect the path detuning. Alternatively,the RF drive signals may be phase-shifted corresponding to the phaseshift for the paths to which they are applied to provide circularpolarization for the coil, the detuning mechanism including circuitrywhich reverses the phase of the RF drive signals.

For an enclosure which is formed of an outer wall, inner wall, and sidewalls, within conductive lands being formed for each wall, correspondinglands on each wall being connected to form the paths, at least the outerwall may have two conductive layers separated by a dielectric, the twoconductive layers each being slotted to form a pattern of lands, withslots on each layer being overlaid by lands of the adjacent layer. Thedegree of overlap for the lands of said layers is at least one factorcontrolling coil resonant frequency.

At least one of the side walls may also have an aperture throughsubstantially the center thereof through which a body to be analyzed maybe passed to an area inside the inner wall, the conductive layer on theinner wall being patterned to provide a selected magnetic flow patternin said aperture. One of the side walls may also be closed, the closedside wall being slotted to form a land pattern covering at least most ofthe wall.

The imaging system may also have at least one gradient coil whichinduces low frequency eddy currents in the RF coil, the slotting on atleast the outer wall and side walls resulting in the breaking up of andsubstantial attenuation of such eddy currents without substantialattenuation of RF currents and fields. The electrical conductor for atleast the outer wall and side walls may be a conductive layer which isthin enough to attenuate low frequency eddy currents while stillconducting RF currents. For such embodiment, the conductor layer has athickness substantially equal to one skin depth at the resonancefrequency to which the coil is tuned, which thickness is substantiallyequal to approximately 5 microns for an illustrative embodiment.

For some embodiments, each of the paths has at least onecircumferential/azimuthal slot formed therein to break the path intosmaller paths. A fixed, variable and/or switched reactive couplingand/or an impedance coupling may be connected across each of thecircumferential slots. Where a reactive coupling is utilized, suchcoupling is a capacitive coupling for illustrative embodiments.

An RF drive signal input is provided to at least one of the paths, thepath inductively coupling an RF drive signal and a path to adjacentpaths.

The dielectric material filling the cavity may provide a selected pathcapacitance, and thus a selected resonant frequency. A mechanism may beprovided for controlling the dielectric fill of the cavity and thus theresonant frequency of the coil.

The electrical conductor may be patterned to form N conductive lands forthe enclosure, each of a selected width, and the number N of conductivepaths and the width of conductive lands for each path may be selected toachieve a desired resonant frequency and a desired field contour.

The enclosure is preferably formed to break induced eddy currents and/orto shape the RF magnetic field patterns.

For some embodiments, a lid is mounted to at least one end of the coil.The lid may be conductive, non-conductive or segmented to be partiallyconductive. A plurality of sample spaces may also be formed in thedielectric at a selected portion of the enclosure, such portion beingone of the side walls for an illustrative embodiment. The sample spacesextend at least part way into the dielectric from the side wall.Alternatively, the open center chamber or aperture of the coil maycontain a dielectric which preferably fills such chamber and a pluralityof sample spaces may penetrate such dielectric. At least a portion of atleast selected ones of the paths may be formed as conductive tubes orcoaxial tube conductors.

For embodiments having a close end wall, the closed end wall functionsas an RF mirror, the end wall having a radial slotting pattern coveringat least most of the wall for an embodiment where the electricalconductors for each wall are slotted.

For some embodiments, field applied to at least one of the electricallyconducting paths causes an alternating magnetic field in the cavity andat least one aperture is formed in at least the electrical conductorthrough which magnetic field may be applied to an adjacent body. Forsome such embodiments, the coil is shaped to conform to the body beingimaged, the surrounding walls including connected top and bottom walls,with the at least one aperture being formed in only the bottom wall. Thecoil may be flexible to conform to a surface of a body being imaged andthe at least one apertures may be arranged to be adjacent the areas tobe imaged of the body being imaged. Where the areas to be imaged are atleast one projection on a body, such projection may extend into thecavity through an adjacent aperture. For such embodiments, thedielectric may be conformable to an outer surface of a projectionextending into the cavity so as to minimize discontinuity between theprojection and the dielectric. Where the coil is formed is a closedloop, an aperture may be formed in only one of an inner or outer wall ofthe coil. Apertures may be arranged to be adjacent areas to be imaged ofa body being imaged.

Various features of the invention, such as the detuning mechanism, maybeemployed independent of other features of the invention. Anotherpotentially independent feature is the dielectric material filling thecavity being utilized to control the resonant frequency for one or moreof the paths. In particular, the dielectric material may be different indifferent areas of the cavity so as to selectively shape the coil field.A mechanism may also be provided for controlling the dielectric fill ofthe cavity and thus the resonant frequency of the coil, for example theamount of fluid in the cavity being controlled where a fluid dielectricis employed. Acoustic damping material may also be provided as a fillfor at least a portion of the cavity. A dielectric material may also beselectively positioned between the coil and a body to be imaged tocontrol and/or shape the field applied to the body from the coil. Thedielectric material preferably substantially fills the space between thecoil and at least a selected area of the body, the dielectric constantof the dielectric substantially matching that of the body in such area.Where a selected area is the area to be imaged, the dielectricconcentrates and directs the field to such area. In its broadest sense,the invention includes a conductive enclosure which is patterned tosuppress low frequency currents and EMI noise.

In accordance with still another aspect of the invention, the RF coilincludes a cavity formed by at least an inner and an outer conductor, adielectric material filling the cavity and at least one sample spaceformed in the coil. The sample space may for example be formed in thedielectric material, projecting therein from a wall of the enclosure.For some embodiments, the coil is a transmission line stub, the innerand outer conductor of the coil being the inner and outer conductors ofthe transmission line stub, respectively. A conductive cap may short oneend of the transmission line stub. The sample space is preferablylocated at a distal end of the stub, the sample space extending fromsuch distal end into the dielectric material or a hollowed-out portionof the center conductor. The stub is tuned and matched so that maximumcurrent, and therefore maximum RF field, occur at such distal end.

The foregoing and other objects, features and advantages of theinvention will be apparent in the following more particular descriptionof preferred embodiments of the invention as illustrated in theaccompanying drawings.

IN THE DRAWINGS

FIGS. 1A and 1B are perspective views of RF coils in accordance withillustrative embodiments of the invention.

FIG. 2A is a sectional view taken generally along the line 2—2 of FIG.1A.

FIG. 2B is the same sectional view as FIG. 2A, but for an alternativeillustrative embodiment.

FIGS. 3A and 3B are illustrations of slotted conductor configurationsfor an outside wall and an inside wall, respectively, of an illustrativecoil having double-sided conductor clad dielectric substrate on bothwalls.

FIGS. 3C and 3D are illustrations of slotted conductor configurationsfor an alternative embodiment.

FIGS. 4A and 4B are illustrations of slotted conductor patterns for theback wall and front wall, respectively, of an illustrative coilembodiment having double-sided conductors on each of these walls and aclosed back wall. The front and back end walls of FIGS. 4A and 4B arepreferably utilized in conjunction with the inside and outside walls ofFIGS. 3A, 3B or of FIGS. 3C3D to form a conductor pattern for an RF coilembodiment.

FIG. 5A is a semi-schematic diagram of a coil for an illustrativeembodiment illustrating a novel drive mechanism for such coil and anovel detuning mechanism for the coil.

FIG. 5B is an enlarged schematic view of a detuning circuit for anembodiment.

FIG. 6 is a top perspective view of a receiver coil illustratingdetuning mechanism.

FIG. 7A and 7B are a top perspective and bottom perspective view,respectively, of an RF coil for another embodiment of the invention.

FIG. 8 is a planar rear view of a coil in accordance with the teachingsof this invention with a patient therein and a field “focusing”dielectric pillow.

FIGS. 9A, 9C and 9E are perspective views of three additionalembodiments of the invention and FIGS. 9B, 9D and 9F are sectional viewsof FIGS. 9A, 9C and 9E, respectively.

FIGS. 10A-10C are perspective views of three coaxial stub embodiments ofthe invention.

FIG. 10D is a cross-sectional view of a generalized coaxial stubembodiment.

DETAILED DESCRIPTION

FIGS. 1A and 2A show an illustrative embodiment of the invention whichovercomes many of the problems discussed earlier. In particular, the RFcoil 10 shown in these figures has a conducting cavity formed as aconductive enclosure 12 in which resonant field can be excited, theenclosure being formed by a surrounding, conducting wall 16, which wallmay be supported by a non-conducting wall 14. Conducting wall 16 may bea whole wall which is at least selectively patterned as described later,or may be formed of conducting tubes, coaxial tubes as in the U.S. Pat.No. 5,557,247 or other appropriate spaced components. Cavity/enclosure12 is filled with air or another dielectric material and supporting wall14 may also be formed of a dielectric material having a dielectricconstant which substantially matches that of the material in cavity 12.Cavity 12 may also be defined by a solid piece of dielectric material ofthe appropriate shape.

The enclosure or cavity 12 may have a circular (FIG. 1B) or elipsoidal(FIG. 1A) transaxial cross-section for many applications, and anelongated axial cross-section, shown for example in FIG. 2A or FIG. 2B,so that the cavity has an outer conducting wall 16O, an inner conductingwall 16I, a front conducting wall 16F, and a rear conducting wall 16R.(1 a,b 5 a,bDisclosure) As indicated above, each of the conducting walls16 maybe supported by a corresponding non-conducting wall 14, or onlyselected ones of the conducting walls may be so supported. In accordancewith the teachings of this invention, each of the conductor walls 16O,16I, 16F, 16R is slotted or otherwise patterned to form a plurality oflands 18 separated by nonconducting slots 20. While in the figures, theslots 20 are shown as being substantially parallel to each other and tothe axis 21 (FIG. 2A) of the coil in each of the inner and outer walls,and continuous with substantially radial slots in the front and rearwalls, this is not a limitation on the invention, and other patterns arepossible. The exact pattern of slots and lands formed for a given coilmay vary with application and with the desired field profile for thecoil. While there is no limit to the width of the slots and lands,narrow slots and wide lands provide better Faraday shielding. Wide slotsand narrower lands allow RF magnetic flux to pass and allow for visual,auditory, physical, and other access to/from the coil's interior throughit's inner and/or outer walls. Thus, for one embodiment, conductingcavity walls 160, 16F, and 16R have narrow slots and wide lands, andconducting wall 16I has narrow lands and wide slots. For thisembodiment, conducting wall 16I may for example be formed of coaxialtube conductors.

One of the objectives of the coils shown in FIGS. 1A ,1B, 2A, and 2B isto suppress eddy currents in the coil, and in particular in the outerwall thereof, caused by the proximity of the RF coil to gradient coilsfor the imaging system, and to suppress other low frequency noise in theconductor, such eddy currents/low frequency noise causing image blurringand therefore adversely affecting the signal-to-noise ratio andresolution achievable from a system employing the coil. One way in whichsuch eddy currents have been suppressed in the past is to have thethickness of the conductor for at least outer wall 16O of the coil, andpreferably for all walls of the coil, thin enough so as to attenuate thelow frequency eddy currents induced by the gradient coil, while stillconducting RF currents. This is possible since the skin depth requiredto conduct signal decreases with increasing frequency, so that if thethickness of the conductor 16 is substantially equal to one skin depthat the resonant frequency to which the coil is tuned, this frequencybeing a relatively high RF frequency, then the conductor will not pass,and will suppress or attenuate the low frequency gradient field inducedsignals or other low frequency noise signals.

However, while this mechanism preserves the coil's RF efficiency whileattenuating switched gradient induced eddy currents, it alone is notsufficient to fully suppress gradient and/or other low frequency noisefor some applications such as fMRI. This objective is facilitated by theslotting or dividing of at least outer wall conducting 16O andpreferably by the slotting of this wall and at least end conductingwalls 16F and 16R. The slotting of the front and rear conducting wallsis desirable to prevent switch gradient induced eddy current flowthrough or around the ends of the coil. The narrower this slotting (i.e.the greater number there are of nonconducting slots 20, and thereforethe narrower the width of each land 18), the more effective this eddycurrent suppression becomes. The combination of the conductor thicknessbeing substantially equal to one skin depth at the resonant frequencyand the slotting of conductor 16, preferably for at least outer wall 16Oand end walls 16F and 16R, provides a substantial elimination of alleddy current induced/low frequency noise in RF coil 10, and thus farclearer images and/or faster imaging, then can otherwise be obtained.

Further, in order to achieve increased field strength to 4T, 7T, 9.4T oreven higher, it is necessary to be able to operate RF coil 10 withincreasingly high frequencies. For example, as previously indicated, fora field strength of 4T, coil 10, when used in an MRI embodiment on thehuman body, must have a resonant frequency of about 170 MHZ, and thisfrequency goes to 400 MHZ for a 9.4T field strength. However, for a coilto resonate at these higher frequencies, the reactance of the coil (i.e.its inductance and capacitance) must be relatively low. Such lowreactances are either not achievable, or are achievable only for coilsso small as to have limited practical application, when lumped inductorsand capacitors are used in conventional lumped element circuit designsfor the coil. Therefore, distributed capacitance and inductance has beenused in distributed element circuit designs to facilitate desired lowerreactances. However, while a coil 10 such as that shown in FIG. 1 withdistributed reactance can offer higher frequency performance than coilsoperating with lumped reactance components, even such head and bodysized coils have difficulty operating with maximum efficiency at Larmorfrequencies corresponding to field strengths above 4 or 5T. The coil 10reduces this problem by breaking the conducting wall to form a largenumber of continuous paths, the lands 18 of the walls being connected orformed into N such conductive paths. Breaking the conducting wall toform N conductive paths also improves the homogeneity of the field, thehigher the value of N, the more homogeneous the field. N may for examplebe 16 or 24. FIG. 1 further shows a circumferential or azimuthal slot 22being formed in for example the center of outer conducting wall 16O,which slot is covered by a collar 24 of a conductive material (thecollar 24 being shown in dotted lines in FIG. 1 so that the structurethereunder may be more easily viewed). The slot 22 further breaks up thepaths, thus shortening the individual paths and reducing the inductancethereof. The greater number of breaks in each of N paths formed aroundcoil 10, the lower the inductance, and thus the higher the resonantfrequency for such coil. Collar 24 may be moved to vary the capacitanceformed by the collar, slot 24 and lands 18 of conductor 16, to vary thecapacitance of each path, and thus fine tune the resonant frequencythereof. Tuning could similarly be performed by variable or fixedcapacitors bridging the gap or gaps in each path. Capacitance for thepaths is also determined by the thickness of dielectric filled cavity12, by the dielectric material in this cavity, by the area and thicknessof conductive wall 16, etc.

Distributed inductance is determined primarily by the uninterruptedconductor length and by the width of each path. Thus, assuming that allpaths are to operate at the same resonant frequency, the slotting ofconductor 16 is selected so that all of the lands 20 are of equal width;however, in applications where different paths are to have differentresonant frequencies, for example every other path having a firstresonant frequency and the remaining paths a second resonant frequency,every other path could be of a first width to provide the first resonantfrequency and the intervening paths of a second width to provide thesecond resonant frequency. Various parameters of the paths may also beselected or adjusted, including capacitance, inductance, phase, andconductor thickness of at least selected walls, to control relativecurrent carrying or otherwise control field contours or profiles withinthe coil.

While only a single circumferential/azimuthal slot 22 on the outer wall16O is shown in FIGS. 1A and 2A, this is not a limitation on theinvention, and higher frequency operation can be achieved by having aplurality of circumferential slots 22 for each path. For example, gapsmight be provided at selected one or more of points A, B, C in FIG. 1B.Further, there are some advantages in having slot 22 on outer wall 16Oof the coil, including the operation of the tuning ring or capacitors 24being easier from this location, and that the E-field applied to apatient/body in the recess 26 formed by inner wall 161 is significantlyreduced, thus reducing detuning caused by the presence of thepatient/body in the coil interior and reducing E-field induced noise andheating in the patient or load. This is a problem particularly at higherfrequencies since more EM energy is lost (radiated) at higherfrequencies for a given coil. However, slots 22 may be at a plurality oflocations on outer wall 16O and/or inner wall 16I, on sidewalls 16Fand/or 16R and/or at the intersections of these various walls. (Seepoints B, C, FIG. 1B). In some applications these azimuthal slots may bebridged by fixed, variable, or switched discrete inductance and/orcapacitance components (see A in FIG. 1B), or by distributed componentssimilar to collar 24, but fixed rather than moveable.

FIG. 2B illustrates a number of variations which are possible inpracticing the teachings of this invention. First, the cavity 12 forFIG. 2A is assumed to be filled with a dielectric, the thickness andother dimensions, including volume, of the cavity and the dielectricconstant of the dielectric being two of the factors which determine aresonant frequency for each path. FIG. 2B illustrates the cavity 12″ asbeing filled with a fluid, for example a liquid or a gas, having a knowndielectric constant. However, a tube 30 connected to a suitable pump mayincrease or decrease the quantity of fluid dielectric in cavity 12″and/or the fluid pressure in the cavity and/or may alter the spacingbetween at least semi-elastic cavity walls. Any of these changes alterthe dielectric constant or volume of the dielectric in cavity 12″, andthus can be used to control the resonant frequency or to tune theresonant frequency of the coil.

Second, the embodiment of FIG. 2B has more clearly defined walls whichmay for example be separately formed as discussed in conjunction withFIGS. 3A-4B. These separate conductive layers may envelop a soliddielectric cavity core or a fluid filled, for example air filled, cavitycore. Corresponding lands on adjacent walls may then be electricallyconnected, for example directly, capacitively, or inductively, to formthe N continuous electrical paths around the coil. While for reasonsindicated above, conducting wall 16 would generally be far too thin toprovide structural integrity for the coil without a supportingdielectric substrate 14, this is not a limitation on the invention andelectrically, all that is required to define cavity 12 is a surroundingconducting wall 16.

While one objective of the invention is to provide distributedcapacitances and inductances to achieve higher frequency and thus higherfield strength operation, in some applications it may be desired tooperate a coil of this invention at a lower resonant frequency to, forexample, permit operation at a lower field strength for a givenapplication, while still achieving the other advantages of thisinvention. Adding discrete reactance, for example the added lumped(fixed, variable, or switched) capacitance (or inductance) elements 32shown in FIG. 2B, may be utilized both to achieve a desired reducedfrequency operation and to tune the paths to a desired lower resonantfrequency. A capacitor 32 could for example be provided for each of theN paths or, where operation at more than one frequency is provided, thecapacitor 32 might only be in the paths to be operated at the lower ofthe frequencies, or the capactance might be one value for one frequency,and another value for another frequency. For example, if every otherpath is to have a resonant frequency at a first higher frequency and theremaining alternate paths to be resonant at a second lower frequency,capacitors 32 might appear only for the alternate paths to be operatedat the second lower frequency, or capacitors of alternating values mayappear on alternating elements to tune two frequencies for the coil.Further, while the embodiment of FIG. 2B has only a single azimuthalslot 34 for each path and a single lumped capacitance or inductanceelement 32, additional azimuthal slots could also be provided for thisembodiment to for example shorten path lengths, and these slots or gapscould be bridged by appropriate lumped reactance elements, lumpedcapacitance elements being shown for bridging gaps 34, or could bebridged by distributed reactance elements such as for the gaps of FIGS.2A, 3B. Except for the differences discussed above, the embodiments ofFIGS. 2A and 2B are otherwise substantially the same and operate insubstantially the same manner to achieve the benefits of this invention.

FIG. 3A illustrates a conducting wall 16 which is typically used as anouter wall 16O, but sometimes used as an inner wall 16I for a coil 10.FIG. 3B illustrates a transmission line conductor 16I which is mostoften used at the inside wall for the volume coils 10 and 10′. Theseconductors differ from the conductors 16 discussed earlier in that eachis made up of an inner layer and an outer layer separated by adielectric to form transmission line elements. For the wall 16 of FIG.3A, the slots for the outer layer are shown in solid lines while theslots for the bottom or inner layer are shown in dotted lines. Thefigure therefore shows these layers as being staggered so that the slotof one layer is overlaid by a land of the other layer. This providessuperior RF efficiency and Faraday shielding for the confinement offields generated by the RF coil. Similarly, for FIG. 3B which shows aconductor preferably used for the inside wall of the coil to formconducting wall 16I, the solid outer conductor is slotted bothlongitudinally and circumferentially, while the dotted bottom conductoris slotted only longitudinally, with lands on each side of the conductoroverlying slots on the opposite side. The double-sided conductors, withlands on the conductor for one side overlying slots for the conductor onthe other side, also applies for the two end walls as illustrated inFIGS. 4A and 4B, FIG. 4A being for example conducting rear wall 16R, andFIG. 4B showing conducting front wall 16F of a coil suitable for examplefor head imaging. While not shown, slots may also be provided in variousof the radial lands for the end walls to shorten path lengths. Thedegree of overlap for lands on opposite sides of each double-sidedconductor controls capacitance, and thus resonant frequency, for theconductive paths. The degree of overlap on the outside and end walls ofthe cavity improve the RF conduction, efficiency and shielding of thecavity. In addition to, or in lieu of, overlapping conductive lands ondouble sided cavity walls, capacitors can be used to bridge the slots orgaps between the lands to provide additional RF conduction or alternateRF paths across these slots.

FIGS. 3C and 3D show a conducting outside wall 16 O′ and a conductinginside wall 16 I′ for an alternative embodiment of the invention. Wall16 O′ is slotted into multiple overlapping layers, as for wall 16 O(FIG. 3A) but with additional azimuthal slotting. Bridging capacitancecan be added across the horizontal azimuthal gaps at for example pointsA, B, and/or C, or anywhere else such gaps appear, and can function totune the coil/path to a desired frequency. Placing lumped or distributedcapacitance at these outside wall positions has the added benefit ofmoving capacitive sections of the coil away from the patient's head orbody. Wall 16 I′ is shown as single-sided printed line segments. Thisinside wall embodiment places the more inductive inside wall surface ofthe coil nearer to the coil load (for example the patient), while themore capacitive outside and/or end walls of the coil are away from theload. The end walls of FIGS. 4A, 4B may be used with the outer and innerwalls 16 O′ and 16 I′ and the walls 16 O, 16 I may be used together orwith other inner/outer wall embodiments depending on application.

FIG. 4A also illustrates another optional feature of the invention inthat the conducting rear wall 16R can be an open wall like FIG. 4B, or aclosed wall, this closed wall acting as an RF mirror which permits thelength of the cavity to be shorter. The shortened coil facilitatesdecreased electrical path lengths for all circuits, thus facilitatinghigher efficiency, higher frequency operation and thus higher fieldstrength operation. In this case, a physically shortened coil is also anelectrically shortened coil (in wavelengths) and therefore a moreefficient coil for a given higher frequency and larger dimension such asa human head sized coil operation at 128 MHz or higher.

Particularly where the coil is being utilized to image the head or brainof a patient, the coil being shortened in this way provides ergonomicbenefits in that it permits at least the patients mouth and sometimesnose/eyes, to be outside of the coil, reducing the claustrophobicfeeling sometimes experienced by patients in such imaging machines, andalso facilitating easier breathing by the patient through the mouth ornose if exposed. This design also permits an optical mirror to bemounted within visual range of the patient to permit visual stimuli tobe provided to the patient, something which is required for variousbrain imaging applications, without requiring a transparent section inthe coil. Since it is necessary to have the coil's current pass througha conductive film or screen covering these transparent regions, thepatient is not afforded an unobstructed view of visual stimuli forexample in some fMRI studies The shortened coil is thus highlyadvantageous. However, if the coil design is such as to extend over thepatient's eyes, patient visibility may be enhanced by providing thinconductors for the conducting wall 16 in areas over the patient's eyes;the conducting wall 16 in this area for example being formed by thin,substantially parallel tubes or coaxial conductors. Alternatively atransparent section such as a view port over the face of a human can beprovided in various ways, including 1) through widened slots (thinconductors) for the inside and outside walls over the face, 2) throughwidened slots or gaps between elements in the inside wall and acontinuous or slotted conductive screen window in the outside wall, or3) through transparent conductive elements, continuous or slotted, forinside and outside walls of the cavity.

As seen in FIG. 4A, the rear wall has a unique slotting pattern, withthe slotting extending over substantially the entire end wall. The rearwall with this unique slotting pattern contributes to the resonantfrequency of each path. The coil with the closed end wall is also moreefficient in that it limits radiation loss which might otherwise occurthrough the end wall, the closed end wall also enhancing coil symmetryand thus facilitating tuning of the paths. This follows from the factthat all of the coil elements or paths are symmetrically referenced tothe same ground plane. Finally, since the patient's mouth can extendbeyond the coil, a bite bar can be provided to reduce patient headmovement, something which facilitates signal averaging and diminishesmotion artifacts. Without the ability to maintain the patient's headabsolutely still, applications involving multiple images, such as forexample fMRI, can provide erroneous results.

FIG. 5A illustrates a number of features of the invention, includingpreferred ways for applying RF drive signals and for outputting receivedRF signals from the coil, and techniques for switched detuning of thecoil in applications where the coil is being used for either transmit orreceive but not both simultaneously. In particular, instead of applyingdrive signal to only one of the paths 18 and relying on inductivecoupling to couple the RF signal to the remaining paths, FIG. 5A show sa technique for applying RF drive signal simultaneously to multiplepaths. In particular, an RF drive (transmit) signal on line 40 is passedthrough transmit/receive switches 42 to a 180° splitter 44 which dividesthe signal into two signals 180° out of phase with each other. Theoutputs from splitter 44 are split again into quadrature hybrids 46A and46B to drive four line elements or paths 18 separated by 90° azimuthangles. Since the electrical phase difference for the signals applied toeach of the paths corresponds to the angular separation between thepaths, the transmit mode is circularly polarized. A second 180° splitter48 combines the quadrature combined receive channels passing through thequadrature hybrids 46 to a common receive channel 50 which is passedthrough switches 42 to receive output line 52. Transmit and receivelines 40, 52 are decoupled from the power amplifier and the preamplifierrespectively by transmit/receive switches 42. In a body coil applicationfor example, these switches will have the attributes of low loss, highspeed and very high power ratings, while requiring low switch biasvoltages. All of the components of the drive circuit should benon-magnetic and can be mounted close to the back of coil 10.

While for the illustrative embodiment shown in FIG. 5A, circularlypolarized drive signals have been applied to four evenly spaced paths oncoil 10 for improved homogeneity, all that is required for circularpolarization, is that the paths to which signal is applied be inrelative quadrature phase. The number of paths coupled electrically tothe power amplifier or power amplifiers and the receiver or receiversmay be any number between 1 and N where N is the number of elements orpaths 18 in FIG. 5A. Thus, assuming 16 paths for an illustrativeembodiment, signal could be applied to 2, 4, 8, or 16 paths for balancedquadrature operation, or could be applied to another number of coils.Transmit signal amplitude, phase angle, and drive paths can be selectedfor maximum homogeneity, or for targeting a desired region of interestin the body or other test object. Similarly, the receive paths and phaseangles can be chosen for overall homogeneity, or for highest sensitivityreception from a specific region of interest. In addition, while signalsare shown applied to a sidewall of the paths, signal can be applied atvarious points on the paths, it being currently preferred that they beapplied to inner conducting wall 16I at a point near its junctions witha sidewall.

Further, the signal on each of the lines is shown as being capacitivelycoupled to the corresponding path through a variable capacitor 54. Whilecapacitive coupling is shown, any reactive coupling (capacitive orinductive) can be used. Operation at two or more frequencies can beachieved for the coils in FIGS. 1, 5 a, by changing the electrical pathlengths for alternating paths for each frequency desired. For example,for a double tuned coil, all odd numbered paths would be adjusted ortuned to one electrical length or frequency. All even numbered pathswould be tuned to a second frequency. This tuning can be achieved byadding or subtracting, inductance or capacitance in the respectivepaths. Operation at two different frequencies may also be achieved byhaving at least one wall of the resonant cavity of greater than one skindepth, and by having a dielectric constant within the cavity differentthan that of air outside of the cavity. This results in a signalwavelength on the inner surface of the conductive cavity wall facing theinner dielectric to be different (in frequency) to the signal wavelengthon the cavity wall facing the outer air dielectric. This results in thecavity being resonant at two different frequencies.

In the discussion so far, it has been assumed that coil 10 is being usedboth as a transmit coil and as a receive coil. However, in someapplications, particularly where homogeneous excitation of, and highsensitivity detection from, localized regions of interest (ROI) isrequired, separate coils may be utilized for transmit and for receive.The coil 10 would more typically be used as a transmit coil, with forexample a phased array or other appropriate receive coil such as isshown in FIG. 6 being placed adjacent the area being imaged on the body.The receive coil may have varying numbers of coil loops of varyingshapes depending on application. In some applications a coil 10 mightonly be used as a receive coil.

One problem when separate transmit and receive coils are used togetheris that destructive reactive coupling may occur between the two coilswhich can interfere with the imaging and eliminate the sensitivitybenefits achievable from having a separate receive coil. It is thereforenecessary to RF field decouple the transmit and receive coils from eachother. This field decoupling can be accomplished by orienting thespatial position of one coil relative to the other, by manipulating theelectrical phase relations of one coil relative to the other, bychanging the field amplitude of one coil relative to the other, bychanging the resonant frequency of one coil relative to the other and/orby temporal separation of the field of one coil relative to the other byany combination of the above techniques. While mechanical means,including relative spatial manipulations of the two coils or mechanicalswitching or reorienting of the phase, amplitude and/or frequency of thecoil, current, voltage and RF fields might be utilized to effect thefield decoupling of the two coils, for preferred embodiments thedecoupling is accomplished electrically or electronically. The actuationor control of such decoupling may be by PIN diodes, solid state switchessuch as transistors, and semiconductor relays, tube switches,electromechanical relays, varistors, etc. In addition to the “active”electronic components indicated above, “passive” components may also beused, including small signal diodes, limiter diodes, rectifier diodes,etc., these components often being used together with quarter-wavecircuits.

Further, by the general methods above, coil coupling can alternativelybe maximized for some applications. For example, it may be desirable forcoil 10 to be strongly coupled to a remote or implanted surface coilwhere transmission-line coupling may be impractical.

Because of its speed, power handling, compactness and non-magneticpackaging, the PIN diode is a good choice for many decoupling circuitimplementations, including ones involving a coil 10. Such PIN diodecircuits can be used to change the electrical length of a coil or itsindividual paths, and to thus change the resonant frequency of one coilrelative to the other coil, decoupling in this case being effected byfrequency shifting. A PIN diode circuit can also be used to open circuitor short circuit a coil or individual paths thereof to effectivelyswitch the coil on or off, thereby decoupling it from the other coil.Similarly, PIN diodes may be used to shift the phase of coil currents tominimize the coupling between two coils.

FIG. 5A shows one way in which a PIN diode 56 maybe utilized to detunethe paths 18 of coil 10, a separate PIN diode switched circuit 56 beingprovided for each path 18 for this embodiment. Each PIN diode shorts twopoints on the corresponding path when conducting, for example a point onan inner wall to a point on a side wall or outer wall, thereby alteringthe effective length, phase or impedance of the path and thus itsresonant frequency.

FIG. 5B shows that this detuning technique can be used with a coil ofthe type shown in U.S. Pat. No. 5,557,247, each transmission lineelement 57 having an inner conductor 59 and an outer conductor 61separated by a dielectric 63. Each PIN diode 56 for this embodiment isconnected through a solder post 60 to short outer conductor 61 throughchoke coil 62 to a conductor 64 on rear wall 68 of the coil. The diodesare current forward biased to short the path, thus dramatically alteringthe coil's resonant frequency, and thus decoupling the coil from anothercoil, a transmit or receive coil of the same operational frequency. Theresonant frequency of each path may be quickly restored by voltage backbiasing the diode to disconnect the conductor 61 from the cavity wall.This PIN diode switching approach is effectively changing the impedanceacross an equivalent gap or azimuthal slot located in the paths atposition “C” in FIG. 1B. This or a similar approach using a PIN diodecircuit to change the impedance (higher or lower) in some or all of thepaths can be affected at other gap positions in the paths such as A orB.

While in FIGS. 5A and 5B, PIN diodes 56 are used to short paths 18 fordetuning, PIN diodes could also be used to effect detuning by placingthe diodes in the path, for example two walls being connected through aPIN diode for each path, the path being open circuited for detuning. ThePIN diodes could be used to quickly switch reactance into or out of eachpath to change its resonant frequency, or the PIN diodes could beutilized to effect detuning in other ways. Further, for the embodimentshown in FIG. 5A, detuning may be effected by reversing the phase forthe RF drive signals applied to the various paths 18 so that electricalphase is out of phase rather than in phase with the azimuthal separationof the paths.

In the discussion so far, coil 10 has been assumed to have a closedtubular configuration with an RF field mode M=1 or greater, so thatfield is applied to a body position within the coil. However, this isnot a limitation on the invention and the coil could be designed tooperate in an M=0 mode for example, as taught in U.S. Pat. No.5,744,957. In particular, by, for example, not slotting the inner wall16I of the coil, or by having a two layer overlapping inner conductor asshown in FIG. 3A, the RF field can be confined in the cavity 12 tocirculate therein, field not exiting the cavity except where an openingis provided in one of the walls of the cavity through which the fieldmay exit or through which a body to be imaged may be inserted into thecavity to be exposed to the field circulating therein. Referring toFIGS. 7A and 7B, the coil 70 may be flat as shown, may have a slightcurvature, may be flat with a circular, elliptical or other appropriateshape rather than a rectangular shape as shown or may have some otherappropriate shape so as to fit on the body being imaged with minimalspacing, thereby achieving optimal coupling between the coil and thebody to be imaged, eliminating losses resulting from dielectric constantmismatches and spaces between the coil and the body. Coil 10 may forexample be flattened to achieve a flat shape. The resonant cavity ofthis invention may thus have a wide variety of sizes and shapes, modesof operation, conductor patterning, apertures, etc. Any cavity coilgeometry is allowable provided that an RF field can be generated thereinwhich can be made useful for MRI or EPR imaging applications. One ormore openings 72 may be provided in a wall of coil 70 which wall is tobe adjacent to or in contact with the body being imaged, each opening 72being adjacent a portion of such body on which imaging is desired. Holesare preferably at B1 magnetic field nodes of the cavity wall. Where theportion of the body on which imaging is desired is a projection on thebody, for example a woman's breasts, opening 72 may be positioned asshown in FIG. 7B to permit such projections to enter cavity 12 throughthe openings so as to be in the field path in the cavity. The dielectricmaterial in the cavity may be shaped or deformable to fit projectionsextending into the cavity, minimizing dielectric (impedance) mismatch.Openings 72 are strategically located and dimensioned to both encompassthe body portions to be imaged and to be properly phased. Openings 72might also be used on an inside or outside wall of a coil 10 designed tooperate in M=0 mode or in a side wall.

In one application, apertures in for example a side wall of a coil areeach dimensioned to hold an experimental mouse, or the mouse's headonly, to permit a plurality of mice to be batch/simultaneously imaged.In particular, referring to FIGS. 1A and 1B, an embodiment of theinvention is shown which is suitable for batch nuclear magneticresonance (NMR) study of multiple laboratory samples, which samples mayfor example be held in test tubes, or of lab animals such as mice. Thecoil 100 is of the type previously described and has a cavity which isfilled with a dielectric material, which material can be a gas such asair, a fluid or a solid. The dielectric material is preferably matchedto the sample to minimize the electrical impedance boundary between thesample and the cavity for improved performance. A solid dielectric canserve as the support wall for sample spaces 102. As for previousembodiments, 16O, 16I, 16F, and 16R are the outer wall, inner wall,front wall and rear wall, respectively, of the cavity. For thisembodiment, these walls may be slotted to break up eddy currents andshield the sample, as for the prior embodiments, or may be continuous.The space or recess 26 inside wall 16I is not used for this embodiment.While end wall 16F may be a conducting wall, for example having slottedconductors between the sample spaces 102 as shown in FIG. 9B, either inaddition to or instead of wall 16F being conductive, a conductive lid104 maybe provided which is mounted to wall 16F in a manner to provide agood electrical connection, for example being press-fitted. The lid aidsthe coil's performance by completely enclosing the coil cavity 12 andthus more efficiently sealing energy in the coil. Where front wall 16Fis conductive, the lid could be non-conductive. The lid could also besegmented, but this would require greater care in mounting the lid toassure the lid segments align with the coil segments.

The embodiment of FIGS. 9C and 9D is substantially the same as theembodiments previously described, for example in conjunction with FIGS.1-4B, and in particular has an inner wall configuration which issubstantially the same as that of FIG. 3C. For this embodiment, theuseful space where a sample would be placed is the center space 26,requiring inner wall 16I to be constructed of conductive lands, tubes ortransmission line elements 18 separated by slots 20 wide enough to allowmagnetic flux to efficiently fill space 26. Outer wall 16O and end walls16F and 16R could be any of the configurations previously described,depending on application. FIGS. 9C and 9D further illustrate the use ofa lid 104 with these embodiments of the invention, which lid may beconductive or non-conductive and serves the same functions as thoseperformed by the lid 104 of FIG. 9A. FIG. 9D also illustrates anoptional solid back wall 106 which also may be continuously conductiveor slotted for the reasons previously discussed.

FIGS. 9E and 9F illustrate still another embodiment of the invention.For this embodiment, center space 26 is filled with a dielectric 108having sample spaces 110 formed therein. Sample spaces 110 may extendcompletely through dielectric 108 or may extend only partly through thedielectric as for spaces 102. While for the embodiment shown, dielectric108 is solid, the gas/air or liquid dielectric is utilized, samplespaces 110 can be formed of non-conducting walls or the sample can beimmersed in the gas or liquid dielectric. For some embodiments, samplespaces 110 may be tubes through which sample passes in a continuous NMRmonitoring process of a gas, liquid or solid sample flow or conveyance.The cavity walls 16 for the embodiment of FIGS. 9E, 9F would besubstantially the same as those for the embodiment of FIGS. 9C, 9D and,as for this prior embodiment, will vary with application. Also, as forthe prior embodiments shown in FIG. 9, the lid 104 is optional,performing the same functions if utilized as for the prior embodiments.

FIG. 10 illustrates several transmission line stub embodiments of theinvention which may for example be used for NMR microprobe applications,permitting small samples to be efficiently measured. Each of thetransmission line stub embodiments 120, 120A-120C, may be capped with aconductive cap 104 to short-circuit its center conductor 122 with itsouter conductor 124. Alternatively, the cap may be spaced or constructedin a manner such that the center conductor and outer conductor of thetransmission line are left open-circuited or the cap may be eliminatedcompletely. The stub is preferably tuned and matched such that themaximum current, and therefore maximum RF magnetic field, is located atthe sample end of the stub. For the current to peak at the end, thelength of the stub should be approximately a one half wavelength or beelectrically adjusted to be, a full wavelength increment of the resonantfrequency for an open stub and one quarter wavelength or three quarterwavelength for a shorted stub. The stub is typically connected to atransmission line to or from the NMR system by a coaxial connector 126which is best seen in cross-sectional view 10D. A variety of coaxialconnectors known in the art might be utilized.

The sample spaces may be located within the dielectric space 128 betweenthe conductors as shown in FIGS. 10A and 10B, or may be located in ahollow, slotted center conductor as shown in FIG. 10C. Moreparticularly, outer wall 124 is the outer conductor or shield of atypical coaxial transmission line. The inner wall is a center conductor122 of a typical coaxial line. The dielectric may be, as for priorembodiments, gas, liquid or solid and impedance matched to the sample.In a typical coaxial line, the dielectric is solid and the sample spacewould be cut or drilled into the dielectric material. The sample space130A for the embodiment of FIG. 10A is the space between the inner andouter conductors at the end of the coil. The sample space 130B for theembodiment of FIG. 10B are multiple sample spaces formed in thedielectric around the center conductor. For the embodiment of FIG. 10C,the center conductor 122 is a slotted or element conductor, the samplespace 130C being within the hollow end of the center conductor.

While in the discussion above it has been assumed that the number oflands on each wall of the coil is the same so that N continuous RFelectrical paths are formed around the coil, this is not a limitation onthe invention. In particular, the number of lands formed on each wall ofthe coil may not be the same. Thus, the outer wall may have a firstnumber of lands N₁ and the inner wall may have a second number of landsN₂. The side walls may also have N₁ lands for reasons previouslyindicated. N₁ may, for example, be selected at least in part toeffectively breakup up low frequency eddy and other currents induced inthe coil, while N₂ is generally selected to achieve a desired magneticfield pattern. Even where N₁ and N₂ are not equal, adjacent paths on thewalls are still connected to form a plurality of continuous electricpaths around the coil, these paths providing various ones of theadvantages previously indicated.

As has been indicated earlier, one advantage of a coil 10 in accordancewith the teachings of this invention is that it can provide a uniform,homogeneous field inside the coil for imaging purposes. While such ahomogeneous field is advantageous in many applications, there areapplications where some other field pattern is desirable. Achieving sucha patterned field through use of spacing and polarization of the pathsto which signals are applied and the phasing of such signals has beendiscussed earlier. The field may also be patterned by the choice,positioning, and control of the dielectric in cavity 12 to obtain adesired field pattern. Still another way of controlling field pattern isillustrated in FIG. 8 where a dielectric “pillow” 80 is shown inside ofcoil 10 which dielectric is not part of cavity 12 and is selected toprovide a good dielectric match with body/patient 82 and/or with thedielectric in cavity 12. The effect of dielectric pillow 80 is toconcentrate or otherwise manipulate the RF magnetic flux in a region ofinterest in the patient's head where, for this embodiment, imaging isdesired. Dielectric inserts could be otherwise positioned between coil10 and the portion of a body on which imaging is to be performed, oreven within cavity 12, to concentrate or manipulate field in such areas,thereby enhancing measurement sensitivity in these regions of imagingand/or to minimize field coupling to areas which are not to be imaged.The shape of the insert, as well as its dielectric constant, is a factorin achieving the desired control and shaping of the Rf field, the shapeof the insert also being useful to restrain body motion, which motion,as previously indicated, can adversely affect imaging.

Another potential problem with MRI and other imaging systems utilizingRF coils is that rapidly switched currents in the field gradients cangenerate intense acoustical noise. Such noise is often annoying to apatient or even painful. One way in which such noise can be reduced isby utilizing an acoustic damping material in cavity 12 as at least partof the dielectric therein, such acoustic dampening material eitherforming the entire dielectric, or being used in conjunction with otherdielectric material in order to achieve a desired dielectric constant orpattern of dielectric constants in the cavity so as to provide a desiredresonant frequency, field pattern and/or other features of theinvention.

Thus, while the invention has been particularly shown and describedabove with reference to illustrative and preferred embodiments, theforegoing and other changes of form and detail may be made therein byone skilled in the art while still remaining within the spirit and scopeof the invention, which is to be defined only by the appended claims.

What is claimed is:
 1. An RF coil for use in an imaging system includinga cavity formed as a conductive enclosure in which resonant fields canbe excited, said conductive enclosure including an electrical conductorpatterned to form a plurality of separate RF conductive paths around thecavity.
 2. An RF coil as claimed in claim 1 including at least onetuning mechanism which determines a resonant frequency for said paths.3. An RF coil as claimed in claim 2, wherein said tuning mechanismreactively adjusts the electrical length of each said path to tune thepath to a said resonant frequency.
 4. An RF coil as claimed in claim 3wherein reactances of said paths are adjusted to achieve a selectedfield profile for the coil.
 5. An RF coil as claimed in claim 3 whereinsaid at least one tuning mechanism tunes all said paths to resonate atthe same frequency.
 6. An RF coil as claimed in claim 3 wherein said atleast one tuning mechanism selectively tunes said paths to resonate atat least two different frequencies.
 7. An RF coil as claimed in claim 6wherein alternate ones of said paths are tuned to resonate at a firstfrequency, and wherein remaining ones of said paths are tuned toresonate at a second frequency.
 8. An RF coil as claimed in claim 2including a dielectric at least substantially filling said cavity.
 9. AnRF coil as claimed in claim 8 wherein the thickness of the conductor forat least selected portions of said enclosure is substantially greaterthan one skin depth at said resonant frequency, and wherein thedielectric filling said cavity has a dielectric constant different fromthat of air, whereby signal of different frequencies propagate on outerand inner surfaces of said conductor.
 10. An RF coil as claimed in claim2 wherein each of said N paths has at least one nonconductive gap formedtherein, and wherein said tuning mechanism includes at least one of areactance and an impedance across at least selected said at least onegap.
 11. An RF coil as claimed in claim 10 wherein saidreactance/impedance for said at least selected gap is variable tocontrol the resonant frequency for the corresponding path.
 12. An RFcoil as claimed in claim 11 wherein said reactance for at least some ofsaid paths includes a capacitor, the capacitance of which may be varied.13. An RF coil as claimed in claim 11 wherein said reactance for atleast some of said paths includes an inductor, the inductance of whichmay be varied.
 14. An RF coil as claimed in claim 11 wherein thevariable impedance/reactance is controlled to at least one of tune,retune and detune the path in which it is located.
 15. An RF coil asclaimed in claim 10 wherein said enclosure is formed of an outer wall,inner wall and side walls, N conductive lands being formed for each saidwall, with corresponding lands on each wall being connected to form saidpaths, and wherein said gap is formed in the conductor for each of saidpaths for at least one of said walls.
 16. An RF coil as claimed in claim15 wherein said gaps are formed in the outer wall conductor for eachpath.
 17. An RF coil as claimed in claim 1 wherein each said paths has aresonant frequency determined by a distributed capacitance and adistributed inductance for the path.
 18. An RF coil as claimed in claim17 wherein said distributed capacitance is determined by at least one ofthe area of the electrical conductor for each path, a dielectric fillmaterial in said cavity, and dimensions of said dielectric fillmaterial.
 19. An RF coil as claimed in claim 17 wherein the electricalconductor forming each path is a thin foil, the distributed inductanceof the path being a function of the path length.
 20. An RF coil asclaimed in claim 17 including at least one reactance component in atleast selected ones of said paths, said reactance component being one ofdistributed and lumped.
 21. An RF coil as claimed in claim 20 whereinsaid at least one discrete reactance is selected to achieve a desiredresonant frequency reduction for the paths.
 22. An RF coil as claimed inclaim 17 wherein each said paths has a cumulative reactance, saidcumulative reactance including at least in part said distributedcapacitance/inductance, the cumulative reactances for the paths beingtuned to result in D different resonant frequencies for the coil, everyD^(th) path symmetrically spaced around the coil being tuned to the samefrequency.
 23. An RF coil as claimed in claim 1 wherein there are Nseparate RF conductive paths around the cavity, and including a circuitwhich applies RF signal to and/or receives RF signals from M selectivelyspaced ones of said paths, where M is an integer and 1≦M≦N.
 24. An RFcoil as claimed in claim 23 wherein the RF signals are phase shiftedcorresponding to a phase shift for the corresponding paths to providecircular polarization for the coil.
 25. An RF coil as claimed in claim23 wherein each RF signal is reactively coupled to the correspondingpath.
 26. An RF coil as claimed in claim 25 wherein coupling reactancefor each path can be varied to independently match/tune the path.
 27. AnRF coil as claimed in claim 26 wherein the coupling reactances areimpedance matched to different loading conditions for the coil.
 28. AnRF coil as claimed in claim 23 wherein said RF coil is used to at leastone of transmit and receive RF signals, but not both simultaneously, andincluding a detuning mechanism for said paths, said detuning mechanismbeing operative when said RF coil is not in the one of transmit/receivemode for which it is being used.
 29. An RF coil as claimed in claim 28wherein said detuning mechanism includes a mechanism for altering atleast one of the path length and impedance for each path to be detuned.30. An RF coil as claimed in claim 29 wherein said detuning mechanismincludes a PIN diode circuit for each path which facilitates rapidswitching to a changed impedance state sufficient to effect the pathdetuning.
 31. An RF coil as claimed in claim 23 wherein the RF drivesignals are phase shifted corresponding to the phase shift for the pathsto which they are applied to provide circular polarization for the coil,and wherein said detuning mechanism includes circuitry which reversesthe phase of the RF drive signals.
 32. An RF coil as claimed in claim 1wherein said enclosure is formed of an outer wall, inner wall and sidewalls, N conductive lands being formed for each said wall, withcorresponding lands on each wall being connected to form said paths,wherein at least said outer wall has two conductive layers separated bya dielectric, and wherein said two conductive layers are each slotted toform a pattern of lands, slots on each layer being overlaid by lands ofthe adjacent layer.
 33. An RF coil as claimed in claim 32 wherein thedegree of overlap for the lands of said layers controls coil resonantfrequency.
 34. An RF coil as claimed in claim 1 wherein said enclosureis formed of an outer wall, inner wall and side walls, N conductivelands being formed for each said wall, with corresponding lands on eachwall being connected to form said paths, and wherein at least one ofsaid side walls has an aperture through substantially the center thereofthrough which a body to be analyzed may be passed to an area inside saidinner wall, the conductive layer on said inner wall being patterned toprovide a selected magnetic flux pattern in said aperture.
 35. An RFcoil as claimed in claim 1 wherein said enclosure is formed of an outerwall, inner wall and side walls, N conductive lands being formed foreach said wall, with corresponding lands on each wall being connected toform said paths, and wherein one of said side walls is closed, saidclosed side wall being slotted to form a land pattern covering at leastmost of said wall.
 36. An RF coil as claimed in claim 1 wherein saidenclosure is formed of an outer wall, inner wall and side walls, Nconductive lands being formed for each said wall, with correspondinglands on each wall being connected to form said paths, wherein saidimaging system has at least one gradient coil which induces lowfrequency eddy currents in said RF coil, and wherein the slotting on atleast said outer wall and side walls results in the breaking up of andsubstantial attenuation of said eddy currents without substantialattenuation of RF currents and fields.
 37. An RF coil as claimed inclaim 36 wherein the electrical conductor for at least said outer walland said side walls is a conductive layer which is thin enough toattenuate said low frequency eddy currents while still conducting saidRF Currents.
 38. An RF coil as claimed in claim 37 wherein said layerhas a thickness substantially equal to one skin depth at the resonantfrequency to which said coil is tuned.
 39. An RF coil as claimed inclaim 37 wherein said layer has a thickness substantially equal toapproximately 5 microns.
 40. An RF coil as claimed in claim 1 whereineach of said paths has at least one circumferential/azimuthal slotformed therein to break said path into smaller subpaths.
 41. An RF coilas claimed in claim 40 including one of a fixed, variable, and switchedreactive coupling and an impedance coupling across each of saidcircumferential slots.
 42. An RF coil as claimed in claim 41 whereinsaid reactive coupling is a capacitive coupling.
 43. An RF coil asclaimed in claim 1 including an RF drive signal input to at last one ofsaid paths, said paths inductively coupling an RF drive signal on a pathto adjacent paths.
 44. An RF coil as claimed in claim 1 including adielectric material filling the cavity, providing a selected pathcapacitance and thus a selected resonant frequency.
 45. An RF coil asclaimed in claim 44 including a mechanism for controlling the dielectricfill of said cavity and thus at least one of the resonant frequency ofthe coil and impedance matching the coil to a body being imaged.
 46. AnRF coil as claimed in claim 1 wherein said electrical conductor ispatterned to form N conductive lands for the enclosure, each of aselected width, and wherein the number N of conductive paths and thewidth of conductive lands for each path are selected to achieve at leastone of a desired resonant frequency and a desired field contour.
 47. AnRF coil as claimed in claim 1 wherein said enclosure at least one ofbreaks induced eddy currents and shapes RF magnetic field patterns. 48.An RF coil as claimed in claim 1 including a lid mounted to at least oneend of the coil.
 49. An RF coil as claimed in claim 48 wherein said lidis at least partially conductive.
 50. An RF coil as claimed in claim 1including a dielectric material filling said cavity, and a plurality ofsample spaces formed in said dielectric at a selected portion of saidenclosure.
 51. An RF coil as claimed in claim 50 wherein said enclosureis formed of an inner wall, an outer wall and side walls, and whereinsaid sample spaces are formed in one of said side walls and extend atleast part way into the dielectric from said side wall.
 52. An RF coilas claimed in claim 1 wherein said coil has an open center chamber andincluding a dielectric in said center chamber, and a plurality of samplespaces penetrating the dielectric.
 53. An RF coil as claimed in claim 1wherein at least a portion of at least selected ones of said paths isformed of one of conductive tubes and coaxial tube conductors.
 54. An RFcoil as claimed in claim 23 wherein the RF signals are phase shifted.55. An RF coil for use in an imaging system including: a cavity formedas a conductive enclosure in which resonant fields can be excited, saidconductive enclosure having N separate continuous RF electrical paths,each of which path has a resonant frequency; and a circuit which appliesRF drive signal to and/or receives RF signals from M selectively spacedones of said paths, where M is an integer and 1≦M≦N.
 56. An RF coil asclaimed in claim 55 wherein the RF signals are phase shiftedcorresponding to a phase shift for the corresponding paths to providecircular polarization for the coil.
 57. An RF coil as claimed in claim55 wherein said tuning mechanism reactively adjusts to achieve aselected field profile.
 58. An RF coil as claimed in claim 55 hereineach RF drive signal is at least one of reactively coupled andreactively decoupled to the corresponding paths.
 59. An RF coil asclaimed in claim 58 wherein coupling reactance for each path can bevaried to at least one of independently tune the path and decouple thepath.
 60. An RF coil as claimed in claim 59 wherein D differentfrequencies are applied to the coil, each frequency being applied toevery D^(th) path evenly spaced along the coil, each of the paths beingtuned to the frequency applied thereto.
 61. An RF coil as claimed inclaim 55 wherein said RF coil is used to at least one of transmit andreceive RF signals but not both simultaneously, and including a detuningmechanism for said paths, said detuning mechanism being operative whensaid RF coil is not in the one of the transmit/receive mode for which itis being used.
 62. An RF coil as claimed in claim 61 wherein saiddetuning mechanism includes a mechanism for altering at least one of thepath length and path impedance for each path to be detuned.
 63. An RFcoil as claimed in claim 62 wherein said detuning mechanism includes aPIN diode circuit which facilitates rapid switching to a changedimpedance state sufficient to effect the path detuning.
 64. An RF coilas claimed in claim 55 wherein the RF drive signals are phase shiftedcorresponding to the phase shift for the paths to which they are appliedto provide circular polarization for the coil, and wherein said detuningmechanism includes circuitry which reverses the phase of the RF drivesignals.
 65. An RF coil as claimed in claim 55 wherein said RF drivesignals are of selected amplitude to provide a desired RF field profile.66. An RF coil as claimed in claim 55 wherein the RF signals are phaseshifted.
 67. An RF coil for use in an imaging system, which coil is usedto at least one of transmit and receive RF signals, but not bothsimultaneously, including: a cavity formed as a conductive enclosure inwhich resonant fields can be excited, said conductive enclosure having Nseparate continuous electrical paths, each of which path has a resonantfrequency; and a detuning mechanism for said paths, said detuningmechanism being operative when said RF coil is not in the one of thetransmit/receive mode for which it is being used.
 68. An RF coil asclaimed in claim 67 wherein said detuning mechanism includes a mechanismfor altering at least one of the RF electrical path length and impedancefor each path to be detuned.
 69. An RF coil as claimed in claim 68wherein said detuning mechanism includes a PIN diode circuit for eachpath which facilitates rapid switching to a changed impedance statesufficient to effect the path detuning.
 70. An RF coil as claimed inclaim 67 wherein RF drive signals are applied to M selectively spacedones of said paths, where M is an integer and 1≦M≦N, and wherein the RFdrive signals are phase shifted corresponding to the phase shift for thepaths to which they are applied to provide circular polarization for thecoil, said detuning mechanism including circuitry which reverses thephase of the RF drive signals.
 71. An RF coil as claimed in claim 67wherein said RF coil is used for transmit only, including a separate RFreceive coil, and a detuning mechanism for said receive coil which isoperative when said receive coil is not in receive mode.
 72. An RF coilas claimed in claim 71 wherein said detuning mechanism for said receivecoil includes a mechanism for altering at least one of path length andimpedances for paths of said receive coil.